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Adsorption-Dissolution Reactions Affecting the Distribution and Stability of CoII EDTA in Iron Oxide-Coated Sand
Abstract
The time-variant chemical behavior of Co[sup II]EDTA (and other metal-EDTA complexes) was investigated in suspensions of iron oxide-coated sand to identify equilibrium and kinetic reactions that control the mobility of Me[sup II]-EDTA complexes in subsurface environments. Batch experiments were conducted to evaluate the adsorption as a function of pH, concentration, and time and to quantify the rate-controlling step(s) of dissolution of the iron oxide by EDTA complexes. Ionic Co[sup 2+] exhibited typical cation-like adsorption, whereas Me[sup II]EDTA adsorption was ligand-like, increasing with decreasing pH. Adsorption isotherms for all reactive species exhibited Langmuir behavior, with site saturation occurring at molar values of
... For instance, it is worth citing: adsorption of Zn II -Pb II /EDTA on clay (Darban et al., 2000); transport of 241 Am 3+ , 60 Co 2+ , 137 Cs + and 85 Sr 2+ in the presence of EDTA through sand/silt/clay soils (Pace et al., 2007, Seliman et al., 2010; mobility of Th IV -EDTA through sand (Reinoso-Maset et al., 2012). The adsorption-dissociation of Co II/III -EDTA during transport has also been reported to occur on iron oxide-coated sand (Szecsody et al., 1994 and1998), on Ferrihydrite (Brooks et al., 1996), and on saprolite (Mayes et al., 2000;Gwo et al., 2007). More recently, a diffusion experiment of 14 C-EDTA was performed through Callovo-Oxfordian (COx) clay rock (Dagnelie et al., 2014). ...
... The interaction of EDTA with minerals potentially dissociates complexes by releasing trace elements such as Fe III , Al III . For example, the adsorption-dissociation of Co II/III -EDTA into Fe III -EDTA was largely studied during percolation through iron oxide-coated sand (Szecsody et al., 1994 and1998). ...
... Slow adsorption induces early break-through during the transport of organic chemicals, especially for large-size humic acids (Pignatello and Xing, 1996). Similar cases were studied for EDTA-lanthanide complexes in Byegard et al. (2000) or for Co II/III -EDTA reactive transport by Szecsody et al. (1994 and1998). In the latter case, the authors needed to modify a parameter from the batch dataset to model EDTA transport (adsorption rates, sites quantities). ...
Adsorption and diffusion experiments of EuIII were performed in Callovo-Oxfordian (COx) clay rock in the presence of EDTA. The predictive model based on binary system parameters (Eu/COx and EDTA/COx) was in good agreement with the results for the Eu/EDTA/COx ternary system. At low EDTA concentrations, the behaviour of EuIII was mainly driven by Eu³⁺ adsorption and complexation by carbonates and EDTA. At higher EDTA concentrations, the behaviour of EuIII was driven by the adsorption of [EuIII-EDTA]⁻ anions. Europium was then used as a probe to estimate the transport of EDTA. Three through-diffusion experiments of EDTA were compared with ¹⁴C, Eu and ¹⁵²Eu tracers. EuIII-EDTA was not quantitatively dissociated by diffusion through the rock. The effective diffusion coefficients quantified De(EuIII-EDTA) = 1.5 - 1.7 10¹² m² s⁻¹ were an order of magnitude lower than that of water, evidencing the anionic exclusion of [EuIII-EDTA]⁻ within the clay rock. Break-through curves and diffusion profiles confirmed retardation due to significant adsorption on the clay rock (Rd(EuIII-EDTA)∼6-14 L kg¹) in comparison with inorganic anions. However, the model based on batch adsorption measurements failed to predict the diffusion results. All experiments displayed an early break-through of EDTA complexes. This behaviour contrasted with results on iron oxides rich sediments, which usually led to higher retardation than expected from the batch studies.
... These reactions include, but are not limited to, sorption of the complex to aquifer minerals, exchange of the metal in the complex by another metal cation, and oxidation/reduction (redox) reactions involving the metal moiety. In contrast to the cationic Co 2÷ species, the anionic complexes of Co-EDTA (Co(II)EDTA 2-and Co(III)EDTA ) exhibit ligand-like sorption behavior, that is, their sorption increases with decreasing pH (Girvin et al., 1993;Szecsody et al., 1994). Thus, past disposal practices that sought to maintain an alkaline environment in the vicinity of waste disposal trenches (Olsen et al., 1983) may have inadvertently contributed to the enhanced migration of EDTA-complexed 6°Co. ...
... In contrast, the dissociation of a metal-ligand complex by a metal with a higher affinity for the ligand can contribute to the delayed transport of the original metal. Szecsody et al. (1994) demonstrated that, in batch experiments, Fe(III) in ferrihydrite displaced Co(II) from adsorbed complexes of Co (II) EDTA 2-resulting in the formation of Fe(III)EDTA-and the release of Co(II) to solution. The aqueous Co(II) was then free to participate in other reactions (e.g., sorption, precipitation) that would result in the loss of Co from solution. ...
... The retardation of both Co-EDTA species decreased with increasing pH of the system ( Fig. 2a and 2b). The decreased travel time for the Co-EDTA species with increased pH is consistent with earlier observations of decreased adsorption with increased pH for Co(II)EDTA 2 on ferrihydrite (Szecsody et al., 1994) and Co(II)EDTA 2-and Co(III)EDTAon 6-A1203 (Girvin et al., 1993). The steady-state concentration of Co(III)EDTA-eluted from the columns increased with decreased pH (Fig. 2b). ...
Many low-level radioactive wastes, historically disposed in shallow land trenches, are illdefined mixtures of radionuclides and organic chelating agents. The observed migration of nuclides, such as 60Co, away from burial sites has been attributed, in part, to the formation of aqueous complexes with ethylenediaminetetraacetic acid (EDTA). The stability of Co-EDTA complexes, and thus the fate and transport: of 60Co in the subsurface, is strongly dependent on the oxidation state of cobalt (log Kco(II)EDTA = 18.3; log KCo(III)EDTA = 43.9). The factors that control the oxidation of Co(II) to Co(III) in subsurface environments are not well understood. We conducted a series of column flow experiments to provide an improved understanding of the geochemical processes that control the reactive transport of cobalt in the subsurface. A solution of 0.2 mM Co(II)EDTA2− in 5 mM CaCl2 was passed through saturated columns that were packed with ferrihydrite (Fe(OH)3)-coated Si02. During transport through the column, a portion of the Co (II) EDTA2− was oxidized to Co (III) EDTA− ; the amount of oxidation reached a steady-state under oxic conditions. Transport of the oxidized species, Co(III)EDTA−, was substantially more rapid than the transport of Co(II) EDTA2−. The retardation of both Co-EDTA species and the extent of cobalt oxidation increased as the pH decreased. These results are consistent with the hypothesis that the association of Co(H)EDTA2− with the ferrihydrite surface is essential for the charge-transfer involved in the oxidation reaction. Co(III)EDTA- exhibited less retardation because this monovalent anion had a lower affinity for the surface than the divalent Co(II)EDTA2−. At faster flow rate, the retardation of Co(II)EDTA2− decreased whereas Co (III) EDTA — breakthrough occurred later; the amount of Co(III)EDTA− formed decreased with increasing flow rate. Under anoxic conditions, the oxidation of Co(II)EDTA2− was decreased, but was not eliminated, suggesting that ferric iron may serve as an oxidant in the system. The loss of oxidative sites under continuous exposure to Co(II)EDTP2− and the blocking of oxidative sites by ions residing on the ferrihydrite surface resulted in a slow decline in the amount of oxidation under anoxic conditions. The oxidation of Co(II)EDTA2− effectively competed with other geochemical reactions such as the Fe(III)-induced dissociation of Co(II)EDTA2− complexes under oxic and anoxic conditions. These results indicate that an iron mineral can be more important for the formation of Co(III)EDTA2− in the subsurface than the mineral is important for the dissociation of Co(II)EDTA− and the concomitant formation of Fe(III)EDTA−. The results suggest that conditions of pH and flow rate that inhibit the formation of the very stable Co(III)EDTA− also promote the undesirable rapid transport of Co(II)EDTA2− posing a challenge to the selection of future waste sites and the development of remedial strategies for existing sites impacted by EDTA-complexed 60Co.
... [26] In spite of the difficulties outlined above, the majority of literature has taken the primitive approach of directly integrating the system of ODEs [e.g., Narasimhan, 1993, 1994;Chilakapati, 1995;Saiers et al., 2000] or the DAE approach [e.g., Miller and Benson, 1983;Chilakapati et al., 1998;Liu et al., 2001]. Neither the primitive nor the DAE approaches will pose computational burdens when the system is small, for example, when modeling laboratory experiments [e.g., Lin and Benjamin, 1990;Szecsody et al., 1994Szecsody et al., , 1998]. However, for large problems or when the reactive chemical model is intended to couple with hydrologic transport, a systematic approach that can overcome the above difficulties is needed. ...
... The reaction network is listed in Table 1. It was simplified by Chilakapati et al. [1998] from the full reaction network of over 64 reactions studied by Szecsody et al. [1994]. Reactions R1 -R5 are fast adsorption/desorption reactions. ...
... For example, R2 (partition between bulk and surface metal ions) is used to reflect the mass balance of surface sites. Some of the reaction constants are obtained from Szecsody et al. [1994] and Yeh et al. [1995]. ...
This paper presents a generic biogeochemical simulator, BIOGEOCHEM. The simulator can read a thermodynamic database based on the EQ3/EQ6 database. It can also read user-specified equilibrium and kinetic reactions (reactions not defined in the format of that in EQ3/EQ6 database) symbolically. BIOGEOCHEM is developed with a general paradigm. It overcomes the requirement in most available reaction-based models that reactions and rate laws be specified in a limited number of canonical forms. The simulator interprets the reactions, and rate laws of virtually any type for input to the MAPLE symbolic mathematical software package. MAPLE then generates Fortran code for the analytical Jacobian matrix used in the Newton-Raphson technique, which are compiled and linked into the BIOGEOCHEM executable. With this feature, the users are exempted from recoding the simulator to accept new equilibrium expressions or kinetic rate laws. Two examples are used to demonstrate the new features of the simulator.
... Soil structure and the properties of the solid soil phase determine these potentials. Pedogenic iron (Fe) minerals (oxides, oxihydroxides, and hydroxides, summarized as Fe (hydr)oxides) play a crucial role for both the stability of soil structure and the surface chemical reactivity of the soil solid phase and can be found ubiquitously in soils and sediments (Sumner 2000). They contribute to the aggregation by cementing clay minerals and quartz particles (Wang et al. 1993) or other soil minerals into stable aggregates (Duiker 2003). ...
... Coatings with hematite present crystals aggregated in spherule shapes, and both Fe minerals are distributed mainly in rough areas of the sand surface (Bedarida et al. 1976;Scheidegger et al. 1993;Ilg et al. 2008;Rusch et al. 2010). Szecsody et al. 1994 reported 2L-FH coating aggregates similar in shape to the ones we obtained, although the coating had isolated aggregates of Fe (hydr)oxide adhering to the surface. In our work, in contrast, the 2L-FH coating covers larger surface areas of the sand grains and has a completely overcast and homogeneous surface after 1 000 min of shaking. ...
Purpose
The production of technosols to remediate polluted or sealed urban soils to sustain new green areas is mainly empirical. For this, our research aims to contribute with the scientific knowledge base for purpose designing of technosols. Since iron minerals play an important role for many different functions of soils, we simplified a technique to incorporate and stabilize iron minerals in a substrate: a sand coated with an amorphous iron (hydr)oxide, a 2-line ferrihydrite (2L-FH).
Materials and methods
The 2L-FH was precipitated by neutralization of a concentrated FeCl3 solution. The suspension was homogeneously mixed with the sand and the mixture was dried at 35 °C. The mechanical stability of the 21 2L-FH-coated sand was determined by shaking the aggregates in water for 0, 1, 10, 100, and 1 000 min. The degree of coating detachment and the properties of the coating after shaking were characterized through (a) Fe content, (b) zeta-potential and particle size of the detached particles, (c) the specific surface area (SSA) of the coated sand, and (d) its surface structure using scanning electron microscopy (SEM). A phosphate adsorption isotherm was performed to measure the P-sorption capacity of the shaken samples and to test the 2L-FH-quartz attachment stability against the surface charge reduction of the 2L-FH associated with P adsorption.
Results and discussion
A reduced Fe loss (30 %) and smaller sizes of the coating detached particles in the sample shaken for 1 000 min indicate that a fractioning and reattachment of these aggregates occurred during the agitation process, resulting in a smoother surface (SEM), and a larger SSA and P-sorption capacity. The coated shaken samples showed P-adsorption capacities (5.3–6.34 μmol P g−1) comparable to high loadings of phosphate in soils, and low detachment of Fe (7–14 %) in spite of negativity surface charge increase.
Conclusions
The practical novel coating process along with the 1 000-min shaking produced a mechanical resistant and P-adsorptive effective coated sand that could sustain the needs of plants in further experiments.
... [Co(III)-EDTA] − is kinetically inert, resistant to ligand exchange reactions (Brooks et al., 1996), and thus promotes Co transport over a long distance. In contrast, [Co(II)-EDTA] 2− can dissociate more readily, allowing free cobalt ions to be sorbed onto oxide minerals, preventing further spreading in natural environment (Szecsody et al., 1994;Gorby et al., 1998;Brooks et al., 1999;Jardine et al., 2002). ...
... Therefore, an improved understanding of the kinetics of [Co(III)-EDTA] − reduction by thermophilic M. thermautotrophicus is important for predicting the fate of 60 Co in such environments. In deep subsurface environments, thermophilic methanogens could use hydrogen as a sole energy source and inorganic carbon as their carbon source (Pedersen, 1999;Kotelnikova, 2002), and in doing so, they could play a vital role in minimizing migration of 60 Co (Szecsody et al., 1994;Gorby et al., 1998;Brooks et al., 1999;Jardine et al., 2002). For that reason, this finding raises the possibility of bioremediation by injecting methanogens into high-temperature subsurface radioactive waste disposal sites, where heavy metals and radionuclides may be the predominant forms of contaminants (Riley et al., 1992;Thomas et al., 1998;Palmisano and Hazen, 2003;Cattant et al., 2008;Gao et al., 2010). ...
Cobalt is a metal contaminant at high temperature radioactive waste disposal sites. Past studies have largely focused on mesophilic microorganisms to remediate cobalt, despite the presence of thermophilic microorganisms at such sites. In this study, Methanothermobacter thermautotrophicus, a thermophilic methanogen, was used to reduce Co(III) in the form of [Co(III)-EDTA]−. Bioreduction experiments were conducted in a growth medium with H2/CO2 as a growth substrate at initial Co(III) concentrations of 1, 2, 4, 7, and 10 mM. At low Co(III) concentrations (< 4 mM), a complete reduction was observed within a week. Wet chemistry, X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) analyses were all consistent in revealing the reduction kinetics. However, at higher concentrations (7 and 10 mM) the reduction extents only reached 69.8% and 48.5%, respectively, likely due to the toxic effect of Co(III) to the methanogen cells as evidenced by a decrease in total cellular protein at these Co(III) concentrations. Methanogenesis was inhibited by Co(III) bioreduction, possibly due to impaired cell growth and electron diversion from CO2 to Co(III). Overall, our results demonstrated the ability of M. thermautotrophicus to reduce Co(III) to Co(II) and its potential application for remediating 60Co contaminant at high temperature subsurface radioactive waste disposal sites.
... Chilakapati et al., 1998;Liu et al., 2001). Neither the primitive nor the DAE approaches will pose computational burdens when the system is small, for example, when modeling laboratory experiments (e. g., Lin and Benjamin, 1990;Szecsody et al., 1994. However, for large problems or when the reactive chemical model is intended to couple with hydrologic transport, a systematic approach that can overcome the above difficulties is needed. ...
... 1. It was simplified byChilakapati et al. [1998] from the full reaction network of over 64 reactions studied bySzecsody et al. [1994]. Reactions R1-R5 are fast adsorption/desorption reactions. ...
A numerical model, HYDROBIOGEOCHEM, is developed for modeling reactive chemical transport under multiphase flow systems. The chemistry part of this model is derived from BIOGEOCHEM, which is a general computer code that simulates biogeochemial processes from a reaction-based mechanistic point of view. To reduce primary dependent variables (PDVs), Gauss-Jordan decomposition is applied to the governing matrix equations for transport, resulting in mobile components and mobile kinetic variables as PDVs. Options of sequential iteration approach (SIA), predictor corrector and operator splitting method are incorporated in the code to make it versatile. The model is a practical tool for assessing migration of subsurface contamination and proper designing of remediation technologies. Examples are presented to demonstrate the capability of the new model.
... During transport, such reactions result in complex changes in 6øCo mobility and spreading due to the differential migration velocities of reaction products (i.e., Co 2+, conEDTA 2-, conIEDTA-). The con/nIEDTA reaction network under pH-buffered conditions (--• 11 reactions) has been well described in batch and one dimensional (l-D) column systems [Szecsody et al., 1994b;Brooks et al., 1996], as modeling simulated experimental data collected over different timescales. However, the oxide phases that largely control metal-EDTA species movement in the subsurface commonly occur as spatially discontinuous inclusions at different scales (centimeters to tens of meters), and the ability to predict transport in these heterogeneous systems is unknown. ...
... The reaction network for the geochemical interaction of ConEDTA 2-with iron oxides and its manifestation in batch systems and 1-D columns is well established by previous studies [Szecsody et al., 1994b (7) and (8)). conEDTA is also oxidized to conIEDTA (reaction (6)) which is a highly stable complex (log K > 40 ] were used to parameterize this reaction network (Table 2) and are the basis for the heterogeneity simulations. ...
Multisolute reactive transport was investigated in chemically heterogeneous systems to determine the influence of the shapes of the reactive heterogeneities (iron oxide inclusions) by comparison of two-dimensional heterogeneous experiments with spatially averaged models that had differing inclusion characterization. Eleven reactions were considered in this system, starting with adsorption of the initial solute (CoIIEDTA) to Fe oxides, followed by two competing surface reactions: oxidation forming CoIIIEDTA and Fe dissolution forming FeIIIEDTA and Co2+. Spatial moments of the eight mobile species were compared between data and models. One spatially averaged model (homogeneous equivalent), which incorporated inclusion mass only, significantly under predicted oxidation (up to 74%), the influence of reaction kinetics, and species retardation. In contrast, the ensemble average model (incorporating inclusion mass and length) well predicted speciation, retardation, and skewness. This large difference in prediction between two spatially averaging models was caused by the lack of incorporation of contact time of solutes with iron oxides in the homogeneous equivalent model and the importance of the contact time with the differing timescales of reactions. Experimental and modeling results also showed that the uncertainty in prediction of specific species increased as the inclusions varied from more ideal (fixed-length) to more natural (variable-length) shape of inclusions.
... Synthetic chelators such as EDTA and NTA have been shown to play an important role in metal/radionuclide speciation, solubility, and mobility in surface waters and groundwater (5)(6)(7)(8)(9)(10)(11). EDTA and NTA complex both Fe(III) and Fe(II) and may therefore influence the microbial reduction of iron(III) oxides by both dissolution (12) and Fe(II) sequestration (our untested hypothesis). ...
... In the case of EDTA, we observed that Fe(II)-EDTA complexes adsorbed to goethite ( Figure 3C). Several metal-EDTA complexes are known to sorb to iron(III) oxides with increasing affinity at lower pH (21)(22)(23), including both Fe(III)-EDTA (7,8,24,25) and Fe(II)-EDTA (13). These later authors observed that Fe(II)EDTA 2complexes, which formed through dissociation of Co(II)EDTA 2in their system, exhibited comparable sorptivity to both the starting and bioreduced goethite mineral. ...
The influence of aqueous (NTA and EDTA) and solid-phase (aluminum oxide, layer silicates) Fe(II) complexants on the long-term microbial reduction of synthetic goethite by Shewanella alga strain BrY was studied. NTA enhanced goethite reduction by promoting aqueous Fe(II) accumulation, in direct proportion to its concentration in culture medium (0.01−5 mM). In contrast, EDTA failed to stimulate goethite reduction at concentrations ≤1 mM, and 5 mM EDTA enhanced the final extent of reduction by only 25% in relation to nonchelator controls. The minor effect of EDTA compared to NTA, despite the greater stability of the Fe(II)−EDTA complex, likely resulted from sorption of Fe(II)−EDTA complexes to goethite. Equilibrium Fe(II) speciation calculations showed that Fe(II)aq should increase with NTA at the expense of the solid-phase Fe(II) species, whereas the opposite trend was true for EDTA due to Fe(II)EDTA adsorption. The presence of aluminum oxide and layer silicates led to a variable but significant (1.5 to > 3-fold) increase in the extent of goethite reduction. Speciation of Fe(II) verified the binding of Fe(II) by these solid-phase materials. Our results support the hypothesis that iron(III) oxide reduction may be enhanced by aqueous or solid-phase compounds which prevent or delay Fe(II) sorption to oxide and FeRB cell surfaces.
... The presence of certain organic ligands is known to reduce the adsorption of cobalt on sediments, minerals, and other geologic materials especially at basic conditions. For example, cobalt adsorption on geologic materials has been shown to decrease by the presence citric acid (Khan et al. 1996), EDTA (Wilding and Rhodes 1963;Cantrell and Serne 1993;Girvin et al. 1993;Jardine et al. 1993;Szecsody et al. 1994Szecsody et al. , 1998Jardine and Taylor 1995;Zachara et al. 1995aZachara et al. , 1995bKhan et al. 1996;Fendorf et al. 1999), hydroxyethylethylenediaminetriacetate acid (HEDTA) , diethylenetriaminepentaacetic acid (DTPA) (Khan et al. 1996), cyclohexanediaminetetraacetic acid (CDTA) (Khan et al. 1996), and nitrilotriacetic acid (NTA) . This decrease in cobalt adsorption is typically caused by the formation of anionic cobalt complexes at near neutral and basic pH conditions, which do not readily adsorb on mineral surfaces at basic pH values. ...
... Since the early 1990s, research has focused primarily on understanding the mechanisms controlling the aqueous speciation and complexation, oxidation/reduction, and sorption of cobalt in the presence of EDTA at the solution/mineral interface (Girvin et al. 1993;Jardine et al. 1993;Szecsody et al. 1994Szecsody et al. , 1998Jardine and Taylor 1995;Zachara et al. 1995aZachara et al. , 1995bZachara et al. , 2000Gorby et al. 1998;Fendorf et al. 1999). This research was prompted by the enhanced migration of 60 Co reported at some DOE sites, such as the Hanford Site (Fruchter et al. 1984(Fruchter et al. , 1985 and Oak Ridge National Laboratory (Means et al. 1976(Means et al. , 1978a). ...
... The presence of certain natural and synthetic organic ligands are known to reduce the adsorption of cobalt on sediments, soils, minerals, and other geologic materials especially at basic conditions. For example, cobalt adsorption on geologic materials has been shown to decrease by the presence citric acid (Khan et al. 1996), oxalic acid (Khan et al. 1996; did not observe a large decrease in cobalt adsorption), 3.9 PNNL-14126 EDTA (Wilding and Rhodes 1963;Cantrell and Serne 1993;Girvin et al. 1993;Jardine et al. 1993;Szecsody et al. 1994Szecsody et al. , 1998Jardine and Taylor 1995;Zachara et al. 1995a,b;Khan et al. 1996;Fendorf et al. 1999), hydroxyethylethylenediaminetriacetate acid (HEDTA) (Delegard and Barney 1983), a diethylenetriaminepentaacetic acid (DTPA) (Khan et al. 1996), cyclohexanediaminetetraacetic acid (CDTA) (Khan et al. 1996), and nitrilotriacetic acid (NTA) (Girvin et al. 1996). The reader should consider this list of complexants (and supporting references) as only examples of some of the organic ligands that affect the fate of cobalt in the environment. ...
... Since the early 1990s, research has focused primarily on understanding the mechanisms controlling the aqueous speciation and complexation, oxidation/reduction, and sorption of cobalt in the presence of EDTA at the solution/mineral interface (Girvin et al. 1993;Jardine et al. 1993;Szecsody et al. 1994Szecsody et al. , 1998Jardine and Taylor 1995;Zachara et al. 1995aZachara et al. ,b, 2000Brooks et al. 1996;Gorby et al. 1998;Fendorf et al. 1999). This research topic was prompted by the enhanced migration of 60 Co reported at some DOE sites, such as the Hanford Site (Fruchter et al. 1984(Fruchter et al. , 1985 and Oak Ridge National Laboratory (Means et al. 1976(Means et al. , 1978aOlsen et al. 1986). ...
In developing the Field Investigation Report (FIR) for the Waste Management Area (WMA) S-SX at the Hanford Site, cesium-137 was the only gamma emitting radionuclide of concern (Knepp 2002). However, in WMA B-BX-BY, the spectral gamma logging data identify seven gamma emitting radionuclides, cesium-137, antimony-125, europium-152 and -154, cobalt-60, uranium-235 and -238 (DOE-GJPO 1998). The geochemical behaviors of several of these radionuclides, antimony-125 and the two europium isotopes, have not been extensively investigated at the Hanford Site. This task was initiated to assure that our understanding of the geochemical properties affecting the environmental behavior of these radionuclides reflects the current state of knowledge. A literature review was conducted to assess the important oxidation/reduction, aqueous speciation, solubility, and adsorption processes affecting the environmental behavior of antimony, cobalt, europium, technetium, and uranium in vadose zone sediments with low-organic matter content in semi-arid environments such as those at the Hanford Site. Technetium-99 was included in this task because of its importance in the long-term risk calculations. This report presents the results of this literature review.
... To etch the surface of the glass beads, grains were immersed in hydrofluoric acid and stirred for 2 hr before rinsing. The iron-oxide coated beads were prepared using the method provided by Szecsody et al. (1994). Acid-washed glass beads were mixed with a ferric oxyhydroxide slurry and allowed to equilibrate for approximately 24hr at pH 7.5. ...
Atomic force microscopy (AFM) and spectral induced polarization (SIP) are widely used to investigate the electrical properties of mineral surfaces at vastly different scales of measurement. We compare AFM and SIP measurements made on two different materials (glass beads and silica gel) subjected to etching, deposition of iron oxide particles, and inclusion of calcite grains. We found that the treatments produced qualitatively consistent behaviors in the AFM and SIP data. Direct AFM measurements of surface charge density for silica and calcite surfaces were quantitatively compared to values estimated from the SIP results using a grain polarization model. No statistically significant difference (at a 95% confidence level) was found between the surface charge density of silica estimated by AFM (2.3 ± 6.6 mC/m² for glass beads and 1.6 ± 0.1 mC/m² for silica gel) versus SIP (5.4 ± 4.4 mC/m² for glass beads and 1.6 ± 0.5 mC/m² for silica gel). The surface charge density for calcite determined by AFM (43.5 ± 12.9 mC/m²) was approximately 19 times higher than that found for silica. While the charge density of calcite surfaces determined by SIP was also generally higher than that found for silica, different treatments produced significantly different values between 4.7 and 258 mC/m² (with a maximum 95% CI of ±8.7 mC/m²). Several possible explanations exist for the range of the observed SIP measurements, including aging of the calcite surfaces. Overall, this study suggests the potential for the complementary use of AFM and SIP measurements to constrain future investigations of polarization mechanisms in porous media.
... Natural aquifers may have different types of heterogeneities across different scales (Babakhani et al., 2017a;Cullen et al., 2010;Murphy et al., 1997;Saiers et al., 1994;Soltanian and Ritzi, 2014;Soltanian et al., 2015a;Soltanian et al., 2017). Such heterogeneities may in general be categorized into five major groups: (i) stratigraphic heterogeneity, most commonly encountered in geological formations, emanates from variation in the type of grains from one layer to another (Babakhani et al., 2018a;Harvey and Garabedian, 1991;Harvey et al., 1993;Huang et al., 2017a;Phenrat et al., 2010a;Soltanian et al., 2015b); (ii) textural heterogeneity, which results from variations in the type of grains within a single-layer aquifer that can be seen at a continuum scale (Babakhani et al., 2017a;Cullen et al., 2010;Saiers et al., 1994;Silliman et al., 2001); (iii) pore-to-continuum heterogeneity emanating from variations in skeletons and pore dimensions, which might lead to preferential flow paths at a pore scale (Babakhani et al., 2017a;Murphy et al., 1997;Szecsody et al., 1994); (iv) nano-and micro-scale heterogeneities, which arise from variations in the surface roughness of porous medium grains affecting particle deposition behaviours Shen et al., 2018); and (v) chemical heterogeneity, which emanates from diversity in surface charge of porous medium grains and might also affect colloid deposition patterns (Elimelech et al., 2000;Knapp et al., 1998;Loveland et al., 2003;Tufenkji and Elimelech, 2005). ...
In order to manage and control the pathogen release from waste streams of various municipal, industrial, and agricultural pollution sources, it is crucial to investigate the impact of release pathways of such contaminants on their fate and transport in groundwater, especially in respect to natural heterogeneities encountered in aquifers. In this laboratory scale study, we investigate the impacts of different release scenarios of Escherichia coli bacteria, including spatially distributed surface recharge and single-point deep injection, as well as mono-pulse and continuous injection on the transport of Escherichia coli within both single-layered and multilayer aquifers.
The results demonstrate earlier arrival of bacteria breakthrough curve (BTC) than conservative solute within a single-layer system with textural and continuum scale heterogeneities, attributed to size exclusion mechanism and preferential flow paths. Size exclusion may be responsible for multiple peaked BTCs observed in all cases of mono-pulse injection of bacteria through both single layer and multi-layer systems. The higher breakthrough of bacteria suspension introduced through a distributed source compared to the point source injection at the same flow rate (19% and 53% in middle and top layers, respectively) suggests that natural hydrologic events such as storm may be more influential in the transport of pathogens in soils than point injections of bacteria in engineering applications such as bioremediation. Moreover, our results reveal that the concentration of the semi-steady state breakthrough formed under distributed and continuous injection condition increases significantly with an increase in the recharge flow rate. This would suggest that a variation in hydrologic conditions can significantly mobilize pathogens which are already deposited in soils.
... An effective technique towards restricting the mentioned technical difficulties is to produce hybrid nanocomposite by saturating or covering the fine particles upon solid particles of larger size. The extensively utilized host materials for nanocomposite construction contains carbonaceous materials like granular activated carbon [2], silica [3], cellulose [4], sands [5,6], and polymers [7,8], and polymeric hosted mainly an attractive choice partially as its mananged pore space and surface chemistry and its outstanding mechanical strength for longstanding usage. The resulting polymerbased nanocomposite (PNC) holds the intrinsic properties of nanoparticles, although the polymer support materials provide higher stability, processability and some interesting improvements caused by the nanoparticle-matrix interaction. ...
This chapter contains sections titled: Introduction Preparation of Polymer Nanocomposites Environmental Applications of PNCs Biocatalytic Nanocomposites Preparation of Nanocellulose Conclusion
... Similar discrepancy was already observed for organic acids/soils systems. For example, it is known that fluid percolation may induce a leaching of surfaces, leading to an increase of adsorption ( Szecsody et al., 1994). Yet, it was surprising in our case that adsorption of carboxylates measured during diffusion was lower than that measured by batch experiments. ...
... Unlike [Co(II)-EDTA] 2− , [Co(III)-EDTA] − is less reactive to exchange cobalt with other metals in solution and is highly mobile in saturated subsurface environments [10]. As compared to [Co(III)-EDTA] − , [Co(II)-EDTA] 2− can easily be dissociated in to Co(II) and EDTA in the presence of cationic metals and the liberated Co(II) can easily be immobilized by the iron oxides [11,12]. The reduction of [Co(III)-EDTA] − can decrease its solubility and thus its migration. ...
Naturally stressed habitats are known to be repositories for novel microorganisms with potential bioremediation applications. In this study, we isolated a [Co(III)-EDTA]− educing bacterium Bacillus licheniformis SPB-2 from a solar salt pan that is exposed to constant cycles of hydration and desiccation in nature. [Co(III)-EDTA]− rted during nuclear waste management process is difficult to remove from the waste due to its high stability and solubility. It's reduced form i.e. [Co(II)-EDTA]2− is less stable though it is toxic. This study showed that B. licheniformis SPB-2 reduced 1 mM [Co(III)-EDTA]− in 14 days when grown in a batch mode. However, subsequent cycles showed an increase in the reduction activity, which was observed up to four cycles. Interestingly, the present study also showed that [Co(III)-EDTA]− acted as an inducer for B. licheniformis SPB-2 spore germination. Vegetative cells germinated from the spores were found to be involved in [Co(III)-EDTA]− reduction. More detailed investigations showed that after [Co(III)-EDTA]− reduction, i.e. [Co(II)-EDTA]2− complex was removed by B. licheniformis SPB-2 from the bulk liquid by adsorption phenomenon. The bacterium showed a D10 value (radiation dose required to kill 90% cells) of ∼250 Gray (Gy), which signifies the potential use of B. licheniformis SPB-2 for bioremediation of moderately active nuclear waste.
... sites, aqueous radionuclides and metals can exist predominantly as anionic EDTA complexes in groundwater (e.g., Means et al., 1978), suggesting that ternary surface complexation could significantly affect contaminant mobilities. A number of studies have focussed on the effects of EDTA on metal sorption (e.g., Girvin et al., 1993;Szecsody et al., 1994); the structure of EDTAbearing surface ternary complexes has been investigated using EXAFS (Schlegel et al., 1997), and several studies have quantified the thermodynamic stabilities of the important ternary surface complexes (Bryce et al., 1994;Zachara et al., 1995;Vohra and Davis, 1998;Yang and Davis, 1999). ...
Humic acid adsorption onto the bacterial surface of Bacillus subtilis was measured with and without Cd, as a function of pH and humic–bacteria–Cd ratios. These experiments tested for the existence of ternary interactions in a bacteria–humic–metal system. We determine both the effects of humic acid on the bacterial adsorption of Cd, as well as the effects of the aqueous metal cation on the bacterial adsorption of humic acid. The presence of Cd does not affect the extent of humic acid adsorption onto the bacterial surface, indicating that there is no competition for sorption sites between humic acid and Cd under the experimental conditions, and that changes in the charging properties of the bacterial surface, as a result of the Cd adsorption, are not significant enough to affect humic acid adsorption.The presence of humic acid does diminish Cd adsorption onto the bacterial surface, suggesting the presence of an aqueous Cd–humate complex under mid to high pH conditions. However, we also observe that the solubility of humic acid is unaffected by the presence of aqueous Cd. This apparently inconsistent behavior of an aqueous Cd–humate complex affecting Cd adsorption but not affecting humic acid solubility is not observed with simpler ionizable organic molecules. We propose that the solubility of humic acid is controlled by the solubility of a less soluble fraction of the acid. Cd forms an aqueous complex with the more soluble fraction of humic acid and there is no interdependence between the aqueous activities of the more and less soluble fractions. That is, the solubility of one humic acid fraction is unaffected by the presence of an aqueous Cd–humate complex involving another humic acid fraction. These experimental results constrain the relative importance of surface ternary and aqueous metal–humate complexes on the bacterial adsorption of both humic acid and metal cations.
... Some sediments may also oxidize Co(II)EDTA to Co(II-I)EDTA, which is highly stable and weakly surface reactive. The geochemical reaction system considered here is applicable to well-buffered groundwater systems [Szecsody et al., 1993] and is a simplification of the full reaction suite of over 64 Figure 3 shows the steady disappearance of Co(II)EDTA, the production/consumption of the dissolution species EDTA and Fe(III)EDTA, and a two-stage growth of microorganisms. Up to 20% of Co(II)EDTA adsorbs immediately onto the surface because of the equilibrium condition reaction (lb). ...
The complex chemistry describing the biogeochemical dynamics in the natural subsurface environments gives rise to heterogeneous reaction networks, the individual segments of which can feature a wide range of timescales. This paper presents a formulation of the mass balance equations for the batch chemistry and the transport of groundwater contaminants participating in such arbitrarily complex networks of reactions. We formulate the batch problem as an initial-value differential algebraic equation (DAE) system and compute its “index” so that the ease of solvability of the system is determined. We show that when the equilibrium reactions obey the law of mass action, the index of this initial-value DAE system is always unity (thus solvable with well-developed techniques) and that the system can be decoupled into a set of linearly implicit ordinary differential equations and a set of explicit algebraic equations. The formulations for the transport of these reaction networks can take advantage of their solvability properties under batch conditions. To avoid the error associated with time splitting fast reactions from transport, we present a split-kinetics approach where the fast equilibrium reactions are combined with transport equations while only the slower kinetic reactions are time split. These results are used to formulate and solve a simplified reaction network for the biogeochemical transformation of Co(II) ethylenediaminetetraacetic acid (EDTA) in the presence of iron-coated sediments.
... The capacity of EDTA to induce and promote the dissolution of iron oxides through surface complex formation that enhance the detachment of the surface metal is well known [23][24][25]. materials can be characterized [21]. ...
Despite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions by elemental iron materials (e.g., in Fe0/H2O systems) are not really understood. One of the main reasons for this is the lack of a unified procedure for conducting batch removal experiments. This study gives a qualitative and semi-quantitative characterization of the effect of the mixing intensity on the oxidative dissolution of iron from two Fe0-materials (material A and B) in a diluted aqueous ethylenediaminetetraacetic solution (2 mM EDTA). Material A (fillings) was a scrap iron and material B (spherical) a commercial material. The Fe0/H2O/EDTA systems were shaken on a rotational shaker at shaking intensities between 0 and 250 min-1 and the time dependence evolution of the iron concentration was recorded. The systems were characterized by the initial iron dissolution rate (kEDTA). The results showed an increased rate of iron dissolution with increasing shaking intensity for both materials. The increased corrosion through shaking was also evidenced through the characterization of the effects of pre-shaking time on kEDTA from material A. Altogether, the results disprove the popular assumption that mixing batch experiments is a tool to limit or eliminate diffusion as dominant transport process of contaminant to the Fe0 surface.
... Amongst the available nano-sized transition metal oxides, nano-Fe 2 O 3 catalyst has the advantages of strong toxicity resistance and high catalytic activity. Recently, many researchers [23][24][25] have produced Fe oxide coatings. Manuel Arruebo [26] has prepared SiO 2 -encapsulated Fe nanoparticles, they found that it was superparamagnetic and with a diamagnetic contribution from the silica shell. ...
The Fe2O3–SiO2 composite (FS) with high specific surface area and pore volume was successfully synthesized using sol–gel method. BET surface area analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), thermal gravity analysis (TGA), temperature programmed desorption (TPD) were used to characterize the catalyst. The performance of prepared FS for elemental mercury removal was evaluated under simulated flue gas. This study identified the effective temperature range (80–450 °C) for elemental mercury removal, with the optimal temperature of 350 °C. A high SO2 (>1500 ppm) concentration is not conducive to elemental mercury removal by FS catalyst. The removal mechanism has been discussed based on the experimental and analytical results.
... In the search for an alternative explanation, ligands have been reported to promote the dissolution of iron oxide through formation of surface complexes that kinetically enhance the detachment. 65 The surface binding constant for CuO-histidine is reported to be higher than that for other amino acids, 66 and the ion concentrations in Figure 2A are well below estimated equilibrium values (see horizontal bars). Both of these facts suggest the different activities among the amino acids are kinetics differences associated with ligand-specific surface binding ability. ...
Copper-based nanoparticles are an important class of materials with applications as catalysts, conductive inks, and antimicrobial agents. Environmental and safety issues are particularly important for copper-based nanomaterials because of their potential large-scale use and their high redox activity and toxicity reported from in vitro studies. Elemental nanocopper oxidizes readily upon atmospheric exposure during storage and use, so copper oxides are highly relevant phases to consider in studies of environmental and health impacts. Here we show that copper oxide nanoparticles undergo profound chemical transformations under conditions relevant to living systems and the natural environment. Copper oxide nanoparticle (CuO-NP) dissolution occurs at lysosomal pH (4-5), but not at neutral pH in pure water. Despite the near-neutral pH of cell culture medium, CuO-NPs undergo significant dissolution in media over time scales relevant to toxicity testing due to ligand-assisted ion release, in which amino acid complexation is an important contributor. Electron paramagnetic resonance (EPR) spectroscopy shows that dissolved copper in association with CuO-NPs are the primary redox-active species. CuO-NPs also undergo sulfidation by a dissolution-reprecipitation mechanism, and the new sulfide surfaces act as catalysts for sulfide oxidation. Copper sulfide NPs are found to be much less cytotoxic than CuO NPs, which is consistent with the very low solubility of CuS. Despite this low solubility of CuS, EPR studies show that sulfidated CuO continues to generate some ROS activity due to the release of free copper by H2O2 oxidation during the Fenton-chemistry-based EPR assay. While sulfidation can serve as a natural detoxification process for nanosilver and other chalcophile metals, our results suggest that sulfidation may not fully and permanently detoxify copper in biological or environmental compartments that contain reactive oxygen species.
... In this study, numerical modeling was used to simulate both batch and column systems with one or multiple reactions. Differential forward and reverse mass flux equations of the species for the reactions considered in each case (Szecsody et al. 1994Szecsody et al. , 1998b) were solved numerically with the fourth-order Runge-Kutta method in batch and a stiff reaction solver method during transport (Hindmarsh 1983). The accuracy of the batch reaction model and submodel of the transport code RAFT (Chilakapati 1995) was tested by comparing it with analytical solutions of a single first-order reaction, a mixed first-second-order reaction, and two reactions (series and parallel). ...
... Most of the studies cited above have been conducted in batch systems using single sorbate-pure phase oxide systems and have significantly advanced our understanding of fundamental processes at the oxide/water interface. Fewer studies have, however, examined sorption behavior organic ligands on minerals with oxide coatings or whole soils ( Kookana and Naidu, 1998;Roden and Zachara, 1996;Szecsody et al., 1994;Zachara et al., 1989;Strek, 1985) and explored competition and displacement effects in multiple solute systems (Ali and Dzombak, 1996;Gu et al., 1996;Vasudevan and Stone, 1996;Balistrieri and Murray, 1987). These topics, although more complex than the simple systems commonly studied, are more closely related to actual soil systems. ...
ABSTRACT This research explores the potential for sorption, desorption, and transformation of three herbicides, 2,4-D,
... As expected, ligand-like adsorption occurred for the free acid (anion) form of NTA or EDTA in suspensions of crystalline Fe oxides (8.5 < pHpz c < 9.3) (Chang et al. 1983). Recent investigations of Co(II)EDTA adsorption on Fe-coated sands (Szecsody et al. 1994) and by Fe and A1 oxide-dominated fluvial sediments (Zachara et al. 1994) showed that Co(II) adsorption in the presence of equal molar EDTA was ligand-like for pH > 7. However, for most materials below pH < 7, competition for the EDTA between Fe or AI (H § promoted dissolution of solids) and Co(II) resulted in the dissociation of the Co(II)EDTA complex . ...
Adsorp tion of Co 2§ nitrilotriacetic acid (NTA) and equal-molar Co 2§ and NTA by a low surface area (LSA) commercial gibbsite (3.5 m 2 g-~) was investigated in batch as a function of pH (4.5 to 10.5), adsorbate (0.5 to 10 txM) and adsorbent (0.5 to 75 g L ~) concentrations and ionic strength (0.01 to 1 M NaC104). The adsorption of Co 2§ (Co-only) and the acid form of NTA (NTA-only) by gibbsite in 0.01 M NaC104 exhibit cation-like and anion-like adsorption edges, respectively. For the equal-molar CoNTA chelate, Co and NTA adsorption edges were similar but not identical to the Co-only and NTA- only edges. Differences suggest the existence of a ternary CoNTA surface complex with the Co in the intact chelate coordinated to surface hydroxyls. NTA-only adsorption was insensitive to ionic strength variation, indicating weak electrostatic contributions to surface coordination reactions. This is consistent with the formation of inner-sphere surface NTA complexes and ligand exchange reactions in which monodentate, bidentate and binuclear NTA surface complexes form. Cobalt adsorption increases (edge shifts to lower pH by 1 pH unit) on LSA gibbsite as ionic strength increases from 0.01 to 1 M NaC104. For the same ionic strength change, a similar shift in the Co-only edge was observed for another com- mercial gibbsite (16.8 m 2 g 1); however, no change was observed for ~-A1203. Ionic strength shifts in Co 2+ adsorption by gibbsite were described as an outer-sphere CoOH § surface complex using the triple- layer model. Results suggest that, at waste disposal sites where 6~ and NTA have been co-disposed, NTA will not promote ligand-like adsorption of Co for acid conditions, but will reduce cation-like ad- sorption for basic conditions. Thus, where gibbsite is the dominant mineral sorbent, NTA will not alter 6~ mobility in acidic pore waters and groundwaters; however, NTA could enhance 6~ mobility where alkaline conditions prevail, unless microbial degradation of the NTA occurs.
... The n potentials of select sediment subsamples from one intact core used in our early study [6], along with clean sand from Unimin Corporation (Unimin Corporation, New Canaan, CT) and Fe coated sand (0.06% Fe, [35]) were measured in NC site groundwater by a streaming potential analyzer equipped with a cylindrical cell and Ag electrodes (Brookhaven, model BI-EKA). The sediment was dominated with quartz, feldspars, clays, and Al and Fe hydroxides in decreasing order of abundance. ...
Our earlier results concerning bacterial transport of an adhesion-deficient strain Comamonas sp. (DA001) in intact sediment cores from near South Oyster, VA demonstrated that grain size is the principle factor controlling bacterial retention, and that Fe and Al hydroxide mineral coatings are of secondary importance. The experimentally determined collision efficiency (α) was in the range of 0.003–0.026 and did not correlate with the Fe and Al concentration. This study attempts to theoretically predict α, and identifies factors responsible for the observed low α. The modified Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to calculate the total intersurface potential energy as a function of separation distance between bacterial and sediment surfaces and to provide insights into the relative importance of bacterial and sediment grain surface properties in controlling magnitude of α. Different models for calculating theoretical α were developed and compared. By comparing theoretical α values from different models with previously published experimental α values, it is possible to identify a suitable model for predicting α. When DA001 bacteria interact with quartz surfaces, the theoretical α best predicts experimental α when DA001 cells are reversibly attached to the secondary minimum of the energy interaction curve and α depends on the probability of escape from that energy well. No energy barrier opposes bacterial attachment to clean iron oxide surface of positive charge at sub-neutral pH, thus the model predicts α of unity. When the iron oxide is equilibrated with natural groundwater containing 5–10 ppm of dissolved organic carbon (DOC), its surface charge reverses, and the model predicts α to be on the order of 0.2. The theoretical α̇ for DA001 in the natural sediments from South Oyster, VA was estimated by representing the surface potential of the sediment as a patch-wise binary mixture of negatively charged quartz (ζ=−60 mV) and organic carbon coated Fe–Al hydroxides (ζ=−2 mV). Such a binary mixing approach generates α that closely matches the experimental α. This study demonstrates that it is possible to predict α from known bacterial and grain surface properties.
This chapter addresses extensive efforts to make a comprehensive review
of the applications of polymer nanocomposites (PNCs) and magnetic
semiconductor photocatalysts (MSPs) for treatment of wastewater. The special
focus has been on the removal of pollutants, such as heavy metal ions, organic
molecules, and waterborne pathogens. The fundamental knowledge for water
treatment and degradation of organic pollutants has been revisited, and the
outcomes in water treatment using PNCs and MSPs have been summarized
with insightful analysis.
Aqueous Fe(II) can induce recrystallization of ferrihydrite and goethite [α-FeOOH] to their more crystalline or molecularly homogenous counterparts. Despite common association with these and other Fe(III) (oxyhydr)oxides, relationships between Fe(II)-induced transformation and Mo mobility remain poorly constrained. We conducted laboratory column experiments to examine repartitioning of sorbed Mo during Fe(II)-induced transformation of ferrihydrite and goethite under advective flow conditions. We first pumped (~0.25 L d⁻¹) artificial groundwater containing 0.1 mM MoO4²⁻ and buffered to pH 6.5 through columns packed with ferrihydrite- and goethite-coated sand until > 90 % Mo breakthrough was observed. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows that initial MoO4²⁻ attenuation resulted from inner sphere complexation of MoO4 tetrahedra at ferrihydrite and goethite surfaces. We then pumped Mo-free anoxic artificial groundwater containing 0.2 mM or 2.0 mM Fe(II) through the columns until effluent Mo concentrations remained < 0.005 mM. Raman spectroscopy shows that Fe(II) introduction induced transformation of both ferrihydrite and goethite to lepidocrocite. Additionally, Fe(II) introduction mobilized 4 to 34 % of sorbed Mo and total mass release was greater for (i) ferrihydrite compared to goethite columns and (ii) low Fe(II) compared to high Fe(II) influent. Effluent pH decreased to ~5.8 for columns receiving the high Fe(II) influent and returned to pH 6.5 after 5 to 10 pore volumes. EXAFS spectroscopy indicates that structural incorporation of MoO6 octahedra into neoformed phases contributes to Mo retention during Fe(II) induced transformation. Our results offer new insight into Mo repartitioning during Fe(II)-induced transformation of Fe(III) (oxyhydr)oxides and, more generally, controls on Mo mobility in geohydrologic systems.
This chapter contains sections titled: Introduction Polymeric Nanocomposites Magnetic Polymer‐Based Nanocomposites Future Perspectives and Conclusion
Contaminants disposed of at the land surface migrate through the vadose zone, forming plumes in groundwater. Processes that occur in the groundwater can attenuate contaminant concentrations during transport through the aquifer. For this reason, quantifying contaminant attenuation and contaminant transport processes in the aquifer, in support of the conceptual site model (CSM) and fate and transport modeling, are important for assessing the need for, and type of, remediation in the groundwater, including monitored natural attenuation (MNA). The framework to characterize attenuation and transport processes provided in U.S. Environmental Protection Agency (EPA) guidance documents was used to guide the laboratory effort reported herein.
Many siderophores and metallophores produced by soil organisms, as well as anthropogenic chelating agent soil amendments, rely upon amine and carboxylate Lewis base groups for metal ion binding. UV-visible spectra of metal ion-chelating agent complexes are often similar and, as a consequence, whole-sample absorbance measurements are an unreliable means of monitoring the progress of exchange reactions. In the present work, we employ capillary electrophoresis to physically separate Ni(II)-tetradentate chelating agent complexes (NiL) from Ni(II)-hexadentate chelating agent complexes (NiY) prior to UV detection, such that progress of the reaction NiL + Y → NiY + L can be conveniently monitored. Rates of ligand exchange for Ni(II) are lower than for other +II transition metal ions. Ni(II) speciation in environmental media is often under kinetic rather than equilibrium control. Nitrilotriacetic acid (NTA), with three carboxylate groups all tethered to a central amine Lewis base group, ethylenediamine-N,N'-diacetic acid (EDDA), with carboxylate-amine-amine-carboxylate groups arranged linearly, plus four structurally related compounds, are used as tetradentate chelating agents. Ethylenediaminetetraacetic acid (EDTA) and the structurally more rigid analog trans-cyclohexaneethylenediaminetetraacetic acid (CDTA) are used as hexadentate chelating agents. Effects of pH and reactant concentration are explored. Ni(II) capture by EDTA was consistently more than an order of magnitude faster than capture by CDTA, and too fast to quantify using our capillary electrophoresis-based technique. Using NiNTA as a reactant, Ni(II) capture by CDTA is independent of CDTA concentration and greatly enhanced by a proton-catalyzed pathway at low pH. Using NiEDDA as reactant, Ni(II) capture by CDTA is first order with respect to CDTA concentration, and the contribution from the proton-catalyzed pathway diminished by CDTA protonation. While the convention is to assign either a disjunctive mechanism or adjunctive mechanism to multidentate ligand exchange reactions, our results indicate that a third “semijunctive” mechanism is necessary to account for slow reactions progressing through L-Ni-Y ternary complexes. Ligand exchange pathways with NTA-type chelating agents are assigned a disjunctive pathway, while pathways with EDDA-type chelating agents are assigned a semijunctive pathway. Based upon operative mechanism(s), magnitudes of exchange rates and effects of ambient geochemical conditions can be predicted.
Nanotechnology is considered to play a key role in shaping current environmental engineering and science in this twenty-first century. The way the nanoscale science research is progressing in the development and use of novel and cost-effective technologies for adsorptive removal, catalytic degradation, and detection of contaminants as well as other environmental concerns is praiseworthy. Polymer nanocomposites (PNCs), which incorporate advantages of both nanoparticles and polymers, have received increasing attention in both academia and industry. They present outstanding mechanical properties and compatibility owing to their polymer matrix, the unique physical and chemical properties caused by the unusually large surface area to volume ratios, and high interfacial reactivity of the nanofillers. In addition, the composites provide an effective approach to overcome the bottleneck problems of nanoparticles in practice such as separation and reuse. This chapter gives an overview of PNCs for environment application. A brief summary of the fabrication methods of PNCs is provided, and recent advances on the application of PNC materials for treatment of contaminants, pollutant sensing and detection, and green chemistry are highlighted. In addition, it also offers a comprehensive discussion on the research trends and prospective in the coming future.
Previous studies show that some radionuclides present in reactor decontamination wastes form strong complexes with the organic complexing agents used to decontaminate reactor cores and piping. Further, the metal-ligand complexes exhibit reduced adsorption to soils. Flow through column tests were used to study the adsorption of metal-organic ligand complexes to two soils. The breakthrough curves for metals and organic ligands for four tests are shown. In all the column tests, the adsorption of the organic ligand-metal complex, or the free organic ligand and free metal (disassociated species) was reversible. That is, close to the total mass injected was recovered in the flushing stage with untraced background solution.
Two tests that used the iron oxide coated sand show complicated behavior that was interpreted as being caused by ligand (both EDTA and picolinate) interacting with the ferric oxides. The Ni-organic ligand complex in both cases appears to exchange Ni for Fe to some extent such that free Ni+2 is produced and Fe (III)-organic complexes are formed that adsorb with different strengths. Further, we suspect that the organic ligand is dissolving some of the ferric oxide coatings and destroying sorption sites. The combination of all these reactions leads to rather complicated breakthrough curves. In tests with picolinate there is partial disassociation of the metal and ligand such that the breakthrough curves for the metal and ligand are different
A recommendation is made to not bury EDTA-laden decontamination wastes with cement. Another observation is that predictions that use simple constant Kd and constant source release constructs may not be exclusively conservative in predicting concentrations of contaminants in water down-gradient from disposal sites.
The migration of ⁶⁰Co from buried radioactive waste in the Hanford subsurface may be facilitated by chelation with organics such as EDTA. The goal of this research was to quantify the rates and mechanisms of Co(II)EDTA²⁻ interactions with twenty Hanford subsurface sediments using kinetic batch experiments. Our results suggested that oxidation to Co(III)EDTA⁻ occurred in all sediments, oxidizing up to 75% of the initial Co(II)EDTA²⁻ concentrations. Oxidation was sustained over 30 days, though rates typically decreased as a function of time. Dissociation of the Co(II)EDTA²⁻ complex and adsorption/precipitation of free Co²⁺ tended to increase with contact time. Correlations were derived between sediment Mn content and 1) the rate of Co(II)EDTA²⁻ disappearance, and 2) the production of Co(III)EDTA⁻. These correlations will provide an improved mechanistic understanding and predictive capability of the interactions of chelated metals in the Hanford subsurface.
Subsurface and surface water contamination at Oak Ridge National Laboratory (ORNL) is extensive due to legacy waste from Cold War era nuclear weapons production. Shallow land burial occurred within a weathered, oxidized and acidic saprolitic material, while the underlying groundwater is dominantly anaerobic and equilibrated to the neutral pH of the bedrock. Consequently, chelated metal transport (Co(II)EDTA²⁻, SrEDTA²⁻, and CdEDTA²⁻) at ORNL is a function of the geochemistry and oxidation state of both the weathered upper unit and the underlying bedrock. In this paper, recent vadose zone and groundwater studies are reviewed and the implications regarding the offsite transport of chelated metals are discussed.
The use of oxide coated sand filtration as an alternative to current costly methods for water treatment has been reviewed. The paper highlights the importance of iron, aluminium and manganese oxides, including sand coated with these oxides, for removal of heavy metals and other impurities from water. The literature review showed that oxide coated sand is able to remove heavy metals (both anions and cations) and other impurities (e.g. natural organic matter, microorganisms, sulphate, manganese, etc.) from surface water or groundwater. In this review an extensive list of the use of oxide (iron, aluminium and manganese) coated sand in water treatment by adsorption and filtration processes has been compiled to provide a summary of available information. The information available on the capacity of coated sand for pollutant uptake is very scarce, however some data on the capacity of coated sand for heavy metals uptake have been compiled and presented. The coated sand could be an alternative emerging technology for water treatment without any side effects or treatment process alterations.
The use of phytoremediation for restoration or amelioration of soils polluted with heavy metals is a promising technique for the near future. Generally, if a plant can accumulate more than 1000 mg kg-1 (or 1000 ppm) of Cu, Co, Cr, Ni, or Pb, or more than 10,000 mg kg-1 (or 10,000 ppm) of Mn or Zn, it is defined as a hyperaccumulator. The discovery and elucidation of the mechanism used by hyperaccumulators to take up, translocate, and fix heavy metals in aerial parts is of great interest to understand the role of such plants in biorestoration. The performance of plants for heavy metals phytoremediation depends on several factors, starting from the bioavailability of metals in the soil, which is conditioned by cation exchange capacity, soil pH and organic matter content, the oxidation state of the metal, presence of both natural and synthetic chelants, interaction with soil microorganisms, etc. The mechanism used by roots to uptake bioavailable metals from the soil is also crucial for the success of bioaccumulation, in addition to mechanisms used to translocate metals from root to shoots after absorption. Significant progress in understanding the mechanisms governing metal hyperaccumulation has been made in the last decade through comparative physiological, genomic, and proteomic studies of hyperaccumulators and related non-hyperaccumulator plants. Parts of these studies are discussed here. Finally, the efficiency in detoxification and sequestration is a key property of hyperaccumulators, allowing them to concentrate huge amounts of heavy metals in aerial organs without apparent phytotoxic effect. This exceptionally high heavy metal accumulation becomes even more surprising considering that it mainly occurs in leaves where photosynthesis, essential for plant survival, is accomplished, and that the photosynthetic apparatus is a major target for most of these contaminants. Although extensive laboratory studies on phytoextraction by plants have been reported, less research has been dedicated to evaluate the performance in field studies, which seems necessary to both validate laboratory studies and fully demonstrate the usefulness of this technique for site restoration. © 2013 Springer-Verlag Berlin Heidelberg. All rights are reserved.
Subsurface migration of 60Co has been attributed to organic chelating agents and oxidation of Co(II)EDTA2- to Co(III)EDTA- by mineral surfaces. Although the oxidized product (Co(III)EDTA-) has been detected in solution, a reduced species has not been measured. As a result, fate and transport mechanisms involving 60Co remain ill-defined. Accordingly, the objective of this research was to determine redox changes in the solid-phase oxidants, β-MnO2 and Fe(OH)3·nH2O, during reaction with Co(II)EDTA2-. Time-resolved changes in the surface composition of β-MnO2 was accomplished in hydrodynamic systems using XANES spectroscopy. The transport of Co(II)EDTA2- through packed beds of β -MnO2 resulted in a decrease in structural Mn(IV) and an increase in Mn(III) as a Mn2O3-like phase. As the quantity of Mn2O3 increased, the production of Co(III)EDTA- decreased. Thus, it appears that the surface association of Mn2O3 produced from the oxidation of Co(II)EDTA2- impedes the redox reaction. We were unable to detect surface structural alterations on Fe(OH)3·nH2O upon reacting with Co(II)EDTA2-.
Oxides of iron are ubiquitous subsurface mineral constituents and control the mobility of metal ions in soils and groundwater by adsorptive retardation. In this laboratory study, iron-oxide-coated sand (IOCS) is used to model lead chemical behavior in soil using batch and column reactor experiments. Several parameters are varied in order to ascertain their effects: pH, ionic strength, complexation by organic ligands, competing cations, and reaction time. In equilibrium experiments, pH was the major factor that controlled the adsorption process, and increasing ionic strength somewhat decreased Pb adsorption below pH = 5 but had no influence at higher pH values. Using a surface complexation, triple-layer model (TLM), an inner-sphere surface reaction was successful in describing the effect of pH on Pb adsorption on IOCS over a range of Pb concentrations at constant ionic strength. EDTA greatly decreased Pb adsorption at equimolar and greater concentrations, with its effect increasing with pH above 6.0, but with a enhanced adsorption at acidic pHs below about 4.5. Soil column studies of Pb adsorption on IOCS are consistent with expectations from batch studies. Results of this study provide guidance in determining conditions that chemically enhance mobilization or immobilization of Pb in soil and groundwater.
Removal behavior for Zn(II) ions from aqueous solutions at micro and tracer concentration levels has been carried out utilizing a 'radiotracer technique' through an adsorption process using synthesized and well characterized sodium titanate as an adsorbent. The study comprises of various physico-chemical parameters viz. concentration, temperature, pH and the effect of some added cations/anions and acid concentrations (HCl/H2SO4) on the adsorption process. The results show that the high uptake of Zn(II) on a sodium titanate surface follows first order rate law and that equilibrium data fit well for a Freundlich adsorption isotherm. A change in temperature (303-333 K) does not markedly affect the uptake of Zn(II) ions. Radiation stability of sodium titanate was also observed by exposing it to neutron and gamma rays from a 11.1 GBq (Ra-Be) neutron source having an integral neutron flux of 3.85 × 106 n/cm2/sec associated with gamma-dose of 1.72 Gy/h also using a gamma-cell (4.66 KGy/h) Co-60 source.
Los experimentos en medio poroso heterogéneo llevados a cabo en tanque de laboratorio se han utilizado con frecuencia como herramienta para la formulación y la validación de aproximaciones a la modelación de diversos fenómenos que se dan en relación con el flujo y transporte en medios porosos. Históricamente, este tipo de modelos tuvieron un cierto auge para la investigación de los fenómenos de dispersión hidrodinámica en medios porosos a partir de mediados del siglo XX. En las dos últimas décadas la mejora de los sistemas de adquisición de datos, la disponibilidad de tecnologías asequibles y precisas de medición de presiones, de conductividades y de procesamiento de imágenes, además de la necesidad de investigar la modelización de problemas no resueltos de forma satisfactoria, han renovado el interés por esta forma de experimentación. Este artículo recoge una síntesis representativa de las investigaciones llevadas a cabo con modelos físicos de laboratorio, llamados ISE (Intermediate Scale Experiments), como opción para la investigación de procesos de flujo y transporte no-reactivo en medios porosos saturados. Se realiza un repaso de las principales características de este tipo de modelos físicos ilustrándose brevemente las características del prototipo de tanque de experimentación desarrollado en la Universidad Politécnica de Valencia para la investigación en 2D de fenómenos de dispersión en medios heterogéneos.
The mechanisms of Co uptake from solution by calcite surfaces have been studied by X-ray photoelectron spectroscopy (XPS) and sputter depth profiling. Two types of calcite samples were examined (finely powdered and individual {104} cleavage surfaces) as a function of pH (7.3–8.9), reaction time (0.9–168 h), and starting aqueous Co concentration (0.01–0.1 mM). In all cases, solutions were saturated with respect to calcite. No evidence was found for oxidation of Co²⁺ upon sorption for either type of sample. Chemical shift and Auger parameter assessment of the sorbed Co²⁺ species indicate that it is most likely present in the form of a (Ca,Co) carbonate solid solution under most conditions tested, although there is some evidence from XPS depth profiling experiments and scanning electron microscopy on the high pH sorption sample that a Co-carbonate or hydroxy carbonate precipitate is also present in minor amounts. More detail on the identity of the sorption reaction products from these experiments can be found in our companion X-ray absorption spectroscopy (XAS) study (Xu et al., 1996).
The sorption of Co(II)EDTA2-- (where EDTA is ethylenediaminetetracetic acid) was investigated on goethite and on eight sand-textured Quaternary and Pliocene fluvial sediments. Dual-label tracer techniques were used to follow the distribution of 60Co(II)-14C/EDTA added as the preformed 1:1, Co(II)EDTA2- complex. Sorption experiments were performed with fixed concentrations of Co(II)EDTA2- (10−5 mol/L) and variable pH (all materials), and fixed pH (4.4) with variable Co(II)EDTA2-- concentrations (two materials), using solids concentrations of 0.5 g/L for goethite and 500 g/L for the sediments and electrolyte concentrations of 0.003 and 0.03 (goethite only) mol/L Ca(Cl04)2. Aqueous Fe3+aq) and Al'(a'q) were measured at the time of the sorption determination. On goethite, Co(II) EDTA2- exhibited anion-like sorption, increasing with decreasing pH. Increasing electrolyte concentration decreased sorption, indicating a weak, ion-pair type surface complex. Below pH6, however, the sorption chemistry of Co2+ and EDTA4- became complex and disparate as a result of Co(II)EDTA2--dissociation. dissociation was driven by exchange with Fe3+(aq). A nonelectrostatic surface complexation model that explicitly considered the Fe3+-Co(II)EDTA2- exchange reaction was able to adequately describe the sorption data using surface complexes with Co(II)EDTA2-, FeEDTA−, and Co2+.
Chemical weathering of iron-bearing rock minerals results in the formation of iron oxides, which can chemically adsorb onto soil surfaces, and/or absorb into their molecular structure. The objective of this paper is to use laboratory-prepared, iron oxide-coated sand to study the role of iron oxide coatings on the small-strain stiffness of coarse-grained soil. Ottawa sands were geochemically coated with iron oxides of goethite and hematite via a heterogeneous suspension reaction in the laboratory. Specimen preparation techniques were chosen to ensure that the soils were not cemented during preparation. SEM images, ICP analysis, and geotechnical index tests were used to characterize the submicron-scaled iron oxide coatings, and the bender element method was used to measure the shear wave velocities of uncoated and the two iron oxide coated sands. Contact mechanics and submicron scale mechanistic approaches were explored to interpret the experimental data, and results indicated that small-strain stiffness of iron oxide coated sands was higher than that of uncoated sands. Results also suggested that a small-strain stiffness behavioral hierarchy associated with iron oxide thermodynamic stability may exist. This study demonstrated that iron oxide coatings significantly influence sand grain-to-grain behavior, even in the absence of cementation or augmented contact area effects, due to an increase in the number of particle contacts present in the iron oxide coated sands.
For environmental investigations, heavy metals are to be analyzed in a variety of natural and technogenic materials like aerosols, dust, water, stream and lake sediments, soils, vegetation or building materials like concrete, bricks, and slags. The choice of which analytical method has to be applied depends on the material to be analyzed, the availability of the method and the goal of the investigation, respectively. This chapter gives an overview on the most important methods. As most analytical methods require a solution, the analysis of environmental materials consists of two or three major steps" first, mechanical sample preparation, like grinding, sieving, drying, or homogenizing, second, sample dissolution or extraction (for solid samples, except for non-destructive methods like X-ray fluorescence), and third, the final instrumental determination. The kind of information, which can be obtained from the analytical results (e.g., whether the bulk concentration of a heavy metal or the speciation can be determined), depends mainly on thesample preparation and sample digestion method. Sample preparation and digestion are main controlling factors influencing the quality of the analytical results. These preconditions are also discussed in the chapter.
The EDTA-enhanced remediation of copper contaminated sandy-loam soil of volcanic origin was investigated. The soil, from an orchard, was contaminated with about 250 mg/kg of copper due to the extensive use of copper sprays. Copper-contaminated soil was packed into 100-mm-long columns, and solutions of Na2H2EDTA with CaCl2, raised to a pH of 6.2, were applied at a flow rate of 24 mm/h. Application of an excess of 0.01 M EDTA leached about half the acid-extractable copper from the soil; most of it coming out in the first 3 liquid-filled pore volumes (PV). Also a 0.5 PV pulse of 0.001 M EDTA was applied to similar soil columns and then either leached immediately with 0.005 M CaCl2, or left for periods of up to 1 month before leaching. With immediate leaching, 70% of the EDTA applied was complexed with copper in the leachate, but after a month's delay only 24% was complexed with copper in the leachate, the rest being complexed with iron. There was no evidence of EDTA retardation or adsorption.
The experimental results were simulated using the convection–dispersion equation, incorporating a source/sink term. This described the competing time-dependent reactions of copper and iron with EDTA, and the reversion of CuEDTA2– to adsorbed Cu2+ and Fe(III)EDTA– in solution. Reasonable simulations were achieved, mostly within errors of observation.
Oxidation-reduction reactions influence the subsurface mobility of a wide variety of toxic contaminants. In the work reported here, we develop a new model for coupled transport and oxidation-reduction, and we test the model against published data on the movement of CoEDTA (cobalt ethylenediaminetetraacetic acid) through columns packed with beta-MnO2-coated sand. The model solves equations for the advective-dispersive transport of three aqueous species: Co(II)EDTA, Co(III)EDTA, and dissolved oxygen. These transport equations are linked with nonlinear kinetics expressions that describe (1) oxidation of Co(II)EDTA to Co(III)EDTA by beta-MnO2, (2) inhibition of Co(II)EDTA oxidation due to precipitation of Mn2O3, an insoluble reaction product that occludes the beta-MnO2 surface, and (3) regeneration of the redox-reactive beta-MnO2 surface through oxidation of the Mn2O3 precipitate by dissolved oxygen. Comparison of experimental and calculated results demonstrates that the model describes the response of the coupled hydrological and geochemical processes to changes in flow rate, influent concentrations of Co(II)EDTA, and beta-MnO2 surface coverages.
Radionuclides and metals can be mobilized by chelating agents typically present in low-level radioactive liquid wastes. [60Co]EDTA in the form 60CoIIIEDTA- represents a highly stable [log KCo(III)EDTA = 43.9] and mobile form of this radionuclide. By contrast, the reduced form of this metal−ligand complex, 60CoIIEDTA2-, is much less stable [log KCo(II)EDTA = 18.3] and less mobile. There is an increasing awareness that dissimilatory metal-reducing bacteria (DMRB) can be used to mediate redox transformations of metals and radionuclides whose stability and mobility are governed by their oxidation state. We conducted a series of column experiments to provide an improved understanding of CoIIIEDTA- reduction by the facultative anaerobe Shewanella alga BrY (BrY). Experiments were conducted under growth conditions using lactate as a carbon and energy source. We were able to demonstrate the sustained reduction of CoIIIEDTA- in column flow experiments with the desired result that a less stable, less mobile product was formed. The amount of reduction varied directly with the fluid residence time in the columns. In the presence of a suitable mineral sorbent [Fe(OH)3], Co-EDTA transport was delayed as a direct consequence of the bacterial reduction reaction. Even in the presence of a strong mineral oxidant (β-MnO2) the net reduction of CoIIIEDTA- dominated the fate and transport of this species. The system was stable after flow interrupts, and metal-reducing activity could be revived after flushing the columns for 5 days with nutrient-free solution. Furthermore, we demonstrated that BrY could grow and carry out sustained reduction using geochemically derived CoIIIEDTA- as terminal electron acceptor. These results demonstrate that DMRB can be effective in the manipulation of redox-sensitive metals and radionuclides in a system characterized by the advective transport of solutes.
Hydrolysis reaction of Fe(NO3)3 at a high temperature in the presence of urea as the homogeneous precipitant was studied. With the prepared ceramic filter balls loaded with α-Fe2O3 after high temperature calcination, the loading of α-Fe2O3 on the porous ceramic filter balls from Fe(NO3)3 solutions of different concentrations and mechanical stability of the loaded α-Fe2O3 were studied. The product was characterized using XRD and SEM. Adsorption experiments were conducted to evaluate the performance of the product in adsorbing NH3-N. It turned out that the specific surface area of the ceramic filter balls loaded with α-Fe2O3 had increased to 36.5387 m2/g from original 4.6127 m2/g. When the concentration of Fe(NO3)3 was 0.40 mol/L, the loading of α-Fe2O3 on the ceramic filter balls accounted for 8.4% of the total mass of the adsorbent and α-Fe2O3 was adsorbed on the filter balls very well. The adsorption isotherm of NH3-N on the ceramic filter ball adsorbent loaded with α-Fe2O3 was of Langmuir type. The saturated adsorption capacity was 3.33 mg/L, and the adsorption constant K was 0.1873. NH3-N was adsorbed by α-Fe2O3 more easily, which was a kind of specific adsorption.
Laboratory experiments were conducted with suspensions of goethite (α-FeOOH) and a subsurface sediment to assess the influence of bacterial iron reduction on the fate of Co(II)EDTA2−, a representative metal-ligand complex of intermediate stability (log KCo(II)EDTA = 17.97). The goethite was synthetic (ca. 55 m2/g) and the sediment was a Pleistocene age, Fe(III) oxide-containing material from the Atlantic coastal plain (Milford). Shewanella alga strain BrY, a dissimilatory iron reducing bacterium (DIRB), was used to promote Fe(III) oxide reduction. Sorption isotherms and pH adsorption edges were measured for Co2+, Fe2+, Co(II)EDTA2−, and Fe(II)EDTA2− on the two sorbents in 0.001 mol/L Ca(ClO4)2 to aid in experiment interpretation. Anoxic suspensions of the sorbents in PIPES buffer at pH 6.5–7.0 were spiked with Co(II)EDTA2− (10−5 mol/L, 60Co and 14EDTA labeled), inoculated with BrY (1–6 × 108 organisms/mL), and the headspace filled with a N2/H2 gas mix. The experiments were conducted under non-growth conditions. The medium did not contain PO43− (with one exception), trace elements, or vitamins. The tubes were incubated under anoxic conditions at 25°C for time periods in excess of 100 d. Replicate tubes were sacrificed and analyzed at desired time periods for pH, Fe(II)TOT, Fe(aq)2+, 60Co, and 14EDTA. Abiotic analogue experiments were conducted where Fe(aq)2+ was added in increasing concentration to Co(II)EDTA2−/mineral suspensions to simulate the influence of bacterial Fe(II) evolution. The DIRB generated Fe(II) from both goethite and the Milford sediment that was strongly sorbed by mineral surfaces. Aqueous Fe2+ increased during the experiment as surfaces became saturated; Fe(aq)2+ induced the dissociation of Co(II)EDTA2− into a mixture of Co2+, Co(II)EDTA2−, and Fe(II)EDTA2− (log KFe(II)EDTA = 15.98). The extent of dissociation of Co(II)EDTA2− was greater in the subsurface sediment because it sorbed Fe(II) less strongly than did goethite. The post dissociation sorption behavior of Co2+ was dependent on pH and the intrinsic sorptivity of the solid phases. Dissociation generally lead to an increase in the sorption (e.g., Kd) of Co2+ relative to EDTA4− (form unspecified). Sorbed biogenic Fe(II) competed with free Co(aq)2+and reduced its sorption relative to unreduced material. It is concluded that cationic radionuclides such as 60Co or 239/240Pu, which may be mobilized from disposed wastes by complexation with EDTA4−, may become immobilized in groundwater zones where dissimilatory bacterial iron reduction is operative.
Amorphous iron oxides in soil are often extracted by an ammonium oxalate solution (Schwertmann, 1964). This treatment may, however, also dissolve crystalline iron oxides and iron silicates (McKeague & Day, 1966; Baril & Bitton, 1969; McKeague et al ., 1971; Arshad et al ., 1972; Pawluk, 1972; Schwertmann, 1973; Taylor & Schwertmann, 1974; Borggaard, 1976).
It has been shown that EDTA can selectively extract amorphous iron oxides from soils (Borggaard, 1979, 1981) and a synthetic mixture of amorphous iron oxide, goethite, and hematite (Borggaard, 1976). As pointed out previously (Borggaard, 1979), the EDTA method should also be tested on selected minerals to decide if it can serve as a reference method against which other less time-consuming methods may be tested.
The dissolution of hematite in the presence of EDTA and of related aminocarboxylic acids has been studied as a function of different parameters. It was found that the leaching of ferric species from this oxide depends greatly on the pH, temperature, and nature of the chelating agents. At lower temperatures (<64°C) EDTA enhances the dissolution of hematite in alkaline solution at pH ≲ 10. As the temperature rises, the leaching of iron is the strongest in acidic solutions. HEDTA and DTPA exhibit behavior rather similar to that of the EDTA, while NTA differs from these complexing agents as it exerts the strongest effect in acidic solutions at all temperatures studied. The results are explained in terms of two different mechanisms: (A) ferric ions released from the bare surface form complexes with chelating ligands in solution, thus causing further leaching of metal ions from the solid, and (B) the chelating species are bound to the lattice metal ions and the complexes are then released into solution. Which of the two mechanisms prevails depends on the conditions of the system and the nature of the chelating agent. At high temperature (215°C) hematite dissolves in acidic solutions with magnetite forming on extended aging. At higher pH values the leaching of iron is diminished and no change of phase is observed.
The adsorption of ethylenediaminetetraacetic acid (EDTA) on a well-characterized sample of α-Fe2O3 (hematite) at 25°C has been studied over a wide range of concentrations and pH. The plateau adsorption density vs pH indicates a maximum at low pH (2–3) and steadily decreasing uptake at pH values above 3. Significant adsorption at the electrokinetic isoelectric point (iep) of hematite (pH ∼7.0) and a shift to lower pH of the iep indicate specific interactions between hematite and the complexing solutes. Essentially the same curve is obtained when the adsorption density relative to its maximum value () is plotted against the pH for EDTA and several other related chelating ligands (HEDTA, DTPA, CTPA, and NTA). A general electrical double-layer model which quantifies the change in chelate ligand solution speciation and the change in particle surface charge is used to model the normalized adsorption () as a function of pH for the five chelating agents. Below the iep the experimental results can be reproduced by adding to the coulombic interaction a specific adsorption energy of approximately −5RT. To fit the data above the iep it was necessary to assume a configurational change for the adsorbed chelating species.
The adsorption of ethylenediaminetetraacetic acid (EDTA) on a well-defined hydrosol consisting of rod-like β-FeOOH particles was studied as a function of pH and temperature. It was found that at 22°C the uptake of EDTA decreases with increasing pH (from pH 2.5 to 9) and it becomes negligible at pH > ∼ 9. At higher temperatures (50 and 90°C) an analogous trend is observed, although the adsorbed amount is smaller at the same pH. The data can be fitted to a Langmuir-type adsorption isotherm, from which the standard adsorption free energy was calculated to decrease from −9.5 to −8.2 kca/mole when the pH rises from 2.8 to 8.2 (at 22°C). Dissolution of ferric ions from the sol particles was determined in the same systems. In the absence of EDTA very small amounts of released Fe3+ ions could be detected over the pH range of 2–13 during 48 hr of equilibration. When EDTA was added little leaching of ferric ions took place up to pH values of ∼7.5 and above 12. In the range of pH 7.5–12 considerable dissolution of Fe3+ was found and the amount of released ferric species increased with increasing concentration of added EDTA. Electrokinetic data showed that the sol had an isoelectric point at pH ∼6.4; however, in the presence of EDTA this was shifted to considerably lower pH values. A mechanism which explains these effects is offered.
The site-binding model for the electrical double layer of hydrous oxides reported in a previous paper is applied to adsorption of anions from dilute solution. Generally, more than one stoichiometric surface reaction is needed to describe the adsorption behavior of divalent weak acid anions. If mass law equations for surface reactions are corrected for effects of the electrostatic field at the interface, the calculated adsorption density depends upon the type of surface species formed. It is shown that calculations which consider formation of surface complexes by protonated anionic forms, e.g., HCrO4−, HSeO4−, HSO4−, are more consistent with experimental adsorption data than complexation by bidentate surface sites. Modeling results predict that adsorbed anions are more easily protonated than those in bulk water, and a qualitative explanation for this phenomenon is presented. The model applies over a wide range of solute concentrations and accounts for effects of changes in composition of the supporting electrolyte. In addition calculated results for a shift in pHPZC upon specific adsorption of sulfate are in reasonable agreement with other experimental studies.
Two multicomponent transport formulations are illustrated with metal leaching in a simple soil analog, i.e., the direct combination of the chemical algebraic equations with the partial differential equations for transport, as well as the separate evaluation of these sets of equations. Exemplary calculations for enhanced metal leaching using a synthetic ligand were done with a general chemical equilibrium program, as a subroutine to the “mixing cell” transport formulation known from systems analysis. A simple criterion for space and time discretization following from numerical dispersion consideration is given. For a non-adsorbing species (ligand) the numerical solution was compared to an analytical solution, to evaluate the accuracy of the program. By appropriate scaling, the numerically obtained approximations for the metal-ligand complexes could also be understood upon comparison with the analytical solution. In order to predict the effect of different feed solution ligand concentrations, a simple analytical approximation was given that showed that time may be scaled linearly with the total ligand concentration. Constraints of this analytical approximation underline the intrinsic multicomponent nature of solute chemistry and transport in soil.
The dissolution kinetics of most slightly soluble oxides and silicates are controlled by chemical processes at the surface. The reaction controlling steps can be interpreted in terms of a surface coordination model.In dilute acid solutions, in the absence of complex-forming ligands, the dissolution kinetics are controlled by the surface bound protons. The rate of the proton-promoted reaction of δ-Al2O3 is RH = kH(CH3)3 where Ch3 is the proton concentration per unit area on the oxide surface. The mechanism can be described by the attachment of three protons to the reaction site prior to the detachment of an Al species into the solution. The dissolution rate of BeO is proportional to (CH2)2. For δ-Al2O3 at pH ⩽ 3.5 dissolution rate is independent of pH; at this pH maximum surface concentration of protons is reached.The organic ligand-promoted dissolution, RL, is of first order with respect to concentration of surface chelates: where {ML} is the concentration of surface chelates per unit area. Detachable surface complexes result from surface coordination of metal ions of the hydrous oxides with bidentate ligands. Especially efficient are bidentate ligands that form mononuclear surface complexes. The sequence of rate constants shows that five- and six-membered chelate rings (oxalate, catechol, malonate and salicylate) enhance the dissolution reactions to a greater extent than seven-membered rings (phthalate, succinate). Monodentate ligands (benzoate ion), though readily adsorbed, do not enhance dissolution rates. However, they can inhibit dissolution by displacing ligands that catalyze this reaction.
The transport and biodegradation of an organic compound (quinoline) were studied in a meter-scale system of layered porous media. A two-dimensional laboratory experiment was conducted in a saturated system with two hydraulic layers with a ratio of conductivities of 1:13. A solution containing dissolved quinoline was injected as a front at one end of the system, and the aqueous-phase concentrations of quinoline, its first degradation product (2-hydroxyquinoline), and oxygen were monitored over time at several spatial locations. Results from a set of ancillary batch and small-column experiments were used to generate a mathematical model for the microbial kinetics; these kinetics described the time rate of change of the concentrations of the two organic compounds (quinoline and 2-hydroxyquinoline), the electron acceptor (oxygen), and microbial biomass. This independently developed kinetic model was incorporated into a two-dimensional numerical model for flow and transport, so that simulations of the laboratory system could be conducted and the results compared with observed data. An analysis of the applicability of single-phase and multiple-phase models for the description of the microbial kinetics was conducted. The results of this analysis indicated that for some cases, it is not necessary to explicitly model the mass transfer between the aqueous phase and the biomass phase. A single-phase model was used for simulating the laboratory system described here. Favorable comparisons between the laboratory and simulation data suggested that a single-phase model was appropriate for describing the microbially mediated reactions in this system. A method for incorporating the effects of metabolic lag into microbial kinetics is described. Metabolic lag was explicitly accounted for in the degradation kinetics for this system; the inclusion of metabolic lag proved to be important for describing transient concentration pulses that were observed in the low-conductivity layer.
This report identifies individual contaminants and contaminant mixtures that have been measured in the ground at 91 waste sites at 18 US Department of Energy (DOE) facilities within the weapons complex. The inventory of chemicals and mixtures was used to identify generic chemical mixtures to be used by DOE's Subsurface Science Program in basic research on the subsurface geochemical and microbiological behavior of mixed contaminants (DOE 1990a and b). The generic mixtures contain specific radionuclides, metals, organic ligands, organic solvents, fuel hydrocarbons, and polychlorinated biphenyls (PCBs) in various binary and ternary combinations. The mixtures are representative of in-ground contaminant associations at DOE facilities that are likely to exhibit complex geochemical behavior as a result of intercontaminant reactions and/or microbiologic activity stimulated by organic substances. Use of the generic mixtures will focus research on important mixed contaminants that are likely to be long-term problems at DOE sites and that will require cleanup or remediation. The report provides information on the frequency of associations among different chemicals and compound classes at DOE waste sites that require remediation.
The adsorption characteristics of a variety of metal-EDTA complexes onto hydrous oxides, principally aluminum oxide (γ-Al2O3), were examined in aqueous solution. Adsorption of these complexes increased with increasing proton concentration due to the formation of surface complexes between EDTA and the surface hydroxo groups, specifically the AlOH2+ surface groups. The pH-dependent adsorptive behavior and the magnitude of adsorption of the "free" EDTA species were similar to those of the metal complexes. The results also showed that the adsorption of "free" EDTA was exothermic, while the adsorption of Ni(II)-EDTA complexes was endothermic in the lower pH region (∼3.5) and exothermic at higher pH values (∼6.0). This implied that the surface preferred the NiHEDTA-1 species rather than the NiEDTA-2 species. Specific adsorption of the metal complexes was evidenced by the charge reversal exhibited by the γ-Al2O3 particles at the highest surface loadings. A quantitative model was formulated based on the pH-dependent speciation of the oxide surface, speciation of the metal complexes in solution, and ζ potential measurements. This model proved valid over a wide range of pH (3-10) and for both high (>50% coverage) and low (<10% coverage) surface loadings.
This paper presents the development and demonstration of a two-dimensional finite-element hydrogeochemical transport model, HYDROGEOCHEM, for simulating transport of reactive multispecies solutes. The model is designed for application to heterogeneous, anisotropic, saturated-unsaturated media under transient or steady state flow conditions. It simulates the chemical processes of complexation, dissolution-precipitation, adsorption-desorption, ion exchange, redox, and acid-base reaction, simultaneously. A set of four example problems are presented. The examples illustrate the model's ability to simulate a variety of reactive transport problems. Important results presented include a depiction of the propagation of multiple precipitation-dissolution fronts, a display of the large errors in model response if the number of iterations between the hydrologic transport and chemical equilibrium modules is limited to one, an illustration of the development of greater concentration of contaminants in groundwater away from a waste site than near the source, and a demonstration of the variation in distribution coefficients of more than 6 orders of magnitude.
The rates of reaction of transition metals with free or protonated ligands are intrinsically fast. Yet the observed rate of formation of metal complexes with strong ligands in seawater containing a mixture of ligands is remarkably slow. When Cu(II) is added to mixtures of natural and synthetic ligands, the equilibrium distribution of metal species may be established only after hours, days, or even months. Initial reaction of the stronger ligand with the added metal is retarded in the presence of seawater concentrations of Ca. Thus Cu complexes are initially formed with the weaker ligands. Final equilibrium metal speciation is attained slowly through a series of ligand- and metal-exchange reactions.Observations of slow kinetics of coordination reactions in model systems demonstrate that re-equilibration of a natural system undergoing perturbations of metal or ligand concentrations, under natural or analytical conditions, cannot be assumed to be rapid. This study also suggests that the concentration of strong complexing agents in seawater may be underestimated in measurements of metal complexation that involve metal additions.
This work compares and models the adsorption of selenium and other anions on a neutral to alkaline surface (amorphous iron oxyhydroxide) and an acidic surface (manganese dioxide). Selenium adsorption on these oxides is examined as a function of pH, particle concentration, oxidation state, and competing anion concentration in order to assess how these factors might influence the mobility of selenium in the environment. The data indicate that 1.1) amorphous iron oxyhydroxide has a greater affinity for selenium than manganese dioxide,2.2) selenite [Se(IV)] adsorption increases with decreasing pH and increasing particle concentration and is stronger than selenate [Se(VI)] adsorption on both oxides, and3.3) selenate does not adsorb on manganese dioxide. The relative affinity of selenate and selenite for the oxides and the lack of adsorption of selenate on a strongly acidic surface suggests that selenate forms outer-sphere complexes while selenite forms inner-sphere complexes with the surfaces. The data also indicate that the competition sequence of other anions with respect to selenite adsorption at pH 7.0 is phosphate > silicate > molybdate > fluoride > sulfate on amorphous iron oxyhydroxide and molybdate ≥ phosphate > silicate > fluoride > sulfate on manganese dioxide. The adsorption of phosphate, molybdate, and silicate on these oxides as a function of pH indicates that the competition sequences reflect the relative affinities of these anions for the surfaces. The Triple Layer surface complexation model is used to provide a quantitative description of these observations and to assess the importance of surface site heterogeneity on anion adsorption. The modeling results suggest that selenite forms binuclear, innersphere complexes with amorphous iron oxyhydroxide and monodentate, inner-sphere complexes with manganese dioxide and that selenate forms outer-sphere, monodentate complexes with amorphous iron oxyhydroxide. The heterogeneity of the oxide surface sites is reflected in decreasing equilibrium constants for selenite with increasing adsorption density and both experimental observations and modeling results suggest that manganese dioxide has fewer sites of higher energy for selenite adsorption than amorphous iron oxyhydroxide. Modeling and interpreting the adsorption of phosphate, molybdate, and silicate on the oxides are made difficult by the lack of constraint in choosing surface species and the fact that equally good fits can be obtained with different surface species. Finally, predictions of anion competition using the model results from single adsorbate systems are not very successful because the model does not account for surface site heterogeneity. Selenite adsorption data from a multi-adsorbate system could be fit if the equilibrium constant for selenite is decreased with increasing anion adsorption density.
The disposal of liquid waste, containing about 0.3 million curies (107 GBq) of fission nuclides, activation products, actinides and transuranics, in a shallow land seepage trench from 1962 to 1966, provided a field opportunity for investigating the chemical, geological, and hydrological processes which affect contaminant migration in soils and weathered bedrock. Gamma-log profiles of wells near the trench indicate that the waste liquids seeped along discrete layers parallel to bedding and along the strikes of faults and folds. Most of the radioactivity measured in the groundwaters consisted of 3H, 99Tc, 60Co, and 233U. The mobility of 99Tc, 60Co, and 233U has been attributed to low molecular weight anionic complexing. Concentrations of 90Sr and 137Cs in the groundwaters were extremely low because of the chemical treatments and precautions taken to establish and maintain an alkaline environment near the trench, which allows for 90Sr sorption and precipitation, and because of the strong tendency for 137Cs to be selectively sorbed by illite. Plutonium isotopic ratios indicate that much of the plutonium contamination near the trench results from the migration of 242Cm and 244Cm and their subsequent decay to 238Pu and 240Pu.Radionuclide concentrations in the groundwaters near the north end of the trench undergo seasonal variations, with the highest activities occurring in the spring and after prolonged rainfall. This suggests that contamination may be leached from the trench or from the relict waste migration layers when the groundwater level rises to saturate these zones or when precipitation infiltrates into the trench or along these relict migration layers during drainage. Suspected transport pathways from the trench to a nearby seep area appear to be associated with fault zones and limbs of a plunging limestone fold.
Nuclide speciation and arboreal uptake have been studied in a forested wetland adjacent to a radioactive waste management area at the Chalk River Nuclear Laboratories in Ontario, Canada. A variety of speciation techniques has revealed the presence of anionic species of several radionuclides in the groundwater. In particular, 60Co exists in solution throughout the wetland predominantly in a very stable, anionic and hydrophilic form. Studies of the concentration of radionuclides in tree sap have shown that there are similarities in the variations in uptake of 60Co, 90Sr and 137Cs with time during the Spring. An experiment using bean plants grown in solution culture with contaminated groundwater spiked with cationic 57Co, demonstrated that the anionic forms of 60Co were less bioavailable than the more labile and cationic forms of cobalt in solution.
The dissolution of goethite and other iron oxides is kinetically controlled by the detachment of an Fe center from the surface. Surface protonation and surface complex formation with ligands are reaction steps preceding the detachment. The dissolution rate is enhanced markedly by oxalate which can form bidentate surface chelates with an Fe center in the oxide surface. An additional catalytic effect is observed under reducing conditions.The electron transfer from the reductant to the surface Fe(III) center precedes the detachment step. This may occur either by direct reduction, subsequent to the adsorption of reductants, or, indirectly by electron transfer from dissolved Fe(II) ions which are generated by the same reductants in solution.
Chelating agents are used in nuclear decontamination operations because they form very selective and strong complexes with numerous radionuclides. However, if environmentally-persistent chelated wastes are disposed of without pretreatment to eliminate the chelating agents, increased radionuclide migration rates from the disposal sites may occur. The environmental chemistry of the three most common aminopolycarboxylic acid chelating agents, NTA (nitrilotriacetic acid), EDTA (ethylenediaminetetraacetic acid), and DTPA (diethylenetriaminepentaacetic acid) is reviewed. This review includes information on their persistence in the environment, as well as their tendency to form complexes with actinides. Data on the sorption of chelated actinides by geologic substrates and on the uptake of chelated actinides by plants are also presented. Increased solubility and/or migration of radionuclides by chelating agents used in decontamination operations have been observed at two different radioactive waste burial grounds. EDTA was found to be promoting the migration of 6OCo and possibly other radionuclides from liquid waste disposal sites at Oak Ridge National Laboratory (1). Recently EDTA has again been identified in radioactive wastes-this time in trench waters containing from 600–16,100 pCi ²³⁸Pu per liter from solid waste burial grounds in Maxey Flats, Kentucky (2). These observations at Oak Ridge and Maxey Flats suggest that the practice of disposing chelated radioactive wastes should be reevaluated. Three different technical options for disposing chelated low-level radioactive wastes are proposed:
After 28 years of continuous use, a low-level liquid radioactive waste infiltration pit at the Chalk River Nuclear Laboratories will soon be decommissioned. In a study undertaken to predict future releases of /sup 60/Co, ratios of sorbed to dissolved /sup 60/Co from 0.5 to greater than 130 mL x g/sup -1/ were found in the underlying aquifer. This ruled out the application of a simple K/sub d/ model for solution/sediment interactions. In subsequent, more detailed, studies a variety of techniques were used to characterize dissolved /sup 60/Co as predominantly (80%) hydrophilic weakly anionic organic complexes of intermediate molecular size. Both naturally occurring and contaminant organics appear to act as complexing ligands. Results of sequential desorption of sediment-bound radiocobalt show that at least three sorption mechanisms (ion exchange, sorption by iron oxide coatings, and ''fixation'') must be involved. On average, more than 80% of the sorbed /sup 60/Co is ''fixed''. Two possible scenarios of future release of sediment-bound /sup 60/Co are discussed. 20 references.
The rate of reaction of inorganic copper with the model ligand ethylenediaminetetraacetic acid (EDTA) is significantly retarded in the presence of calcium at seawater concentrations. The (pseudo-first-order) half-life for inorganic copper reacting with EDTA in seawater is ∼ 2 h at 10-7 M EDTA. This kinetic hindrance to the formation of the thermodynamically favored CuEDTA species results from several factors: (1) the preponderance of the calcium complex in the speciation of EDTA, (2) the competition of calcium and copper for reaction with any free EDTA formed by the dissociation of CaEDTA, and (3) the slow kinetics of direct attack of copper on CaEDTA compared to reaction with free or protonated EDTA species. If metal-complexing agents in natural waters behave as discrete ligands, then the reaction of a metal at strong binding sites may also be kinetically hindered at high alkaline-earth concentrations. In contrast with the reaction of EDTA, however, the rate of complexation of copper by humic acid is not observably affected by high calcium concentrations.
Ligand-exchange reactions of copper-nitrilotriacetate (CuNTA) and copper-humate complexes with a fluores- cent ligand, calcein, were examined and compared. The reverse reactions of copper-calcein with ethylenedi- aminetetraacetate (EDTA) and NTA were also studied; the correspondence of equilibrium constants and kinetic rate constants is discussed. Reactions with model ligands proceeded both by complete dissociation of the initial complex (disjunctive pathway) and by direct attack of the incoming ligand on the initial complex (adjunctive path- way). Reactions of humate-bound copper were also con- sistent with this mechanistic interpretation. The contri- butions of the adjunctive and disjunctive pathways de- pended on the Cu-to-humate loading; both pathways were important at the Cu-to-DOC loadings typical of natural waters. At higher values, disjunctive ligand exchange predominated. This pathway could be interpreted in terms of dissociation of Cu from a humate-binding site and the rate constant related to the conditional stability constant for Cu-humate binding. In this study, we have investigated rates of complex dissociation and ligand exchange for copper under envi- ronmentally relevant conditions of pH and metal and ligand concentrations. We have determined the predom- inant mechanistic pathways for exchange reactions with model ligands under such conditions, focusing on the ef- fects of the relative concentrations of the competing ligands and of the metal-to-ligand ratios. We then examined the kinetics of ligand-exchange reactions of humate-bound copper. These reactions can be described in terms of the mechanisms applicable to the model ligands.
Chromate adsorption on amorphous iron oxyhydroxide was investigated in dilute iron suspensions as a single solute and in solutions of increasing complexity containing CO2(g), SO4S (aq), H4SiO4(aq), and cations (K , MgS , CaS (aq)). In paired-solute systems (e.g., CrO4S -H2CO3*), anionic cosolutes markedly reduce CrO4S adsorption through a combination of competitive and electrostatic effects, but cations exert no appreciable influence. Additionally, H4SiO4 exhibits a strong time-dependent effect: CrO4S adsorption is greatly decreased with increasing H4SiO4 contact time. In multiple-ion mixtures, each anion added to the mixture decreases CrO4S adsorption further. Adsorption constants for the individual reactive solutes were used in the triple-layer model. The model calculations are in good agreement with the CrO4S adsorption data for paired- and multiple-solute systems. However, the model calculations underestimate CrO4S adsorption when surface site saturation is approached. Questions remain regarding the surface interactions of both CO2(aq) and H4SiO4. The results have major implications for the adsorption behavior of CrO4S and other oxyanions in subsurface waters.
Ion exchange, gel filtration chromatography, and gas chromatographymass spectrometry analyses have demonstrated that ethylenediaminetetraacetic acid (EDTA), an extremely strong complexing agent commonly used in decontamination operations at nuclear facilities, is causing the low-level migration of cobalt-60 from intermediate-level liquid waste disposal pits and trenches in the Oak Ridge National Laboratory burial grounds. Because it forms extremely strong complexes with rare earths and actinides, EDTA or similar chelates may also be contributing to the mobilization of these radionuclides from various terrestrial radioactive waste burial sites around the country.
Plutonium in trench leachates at the Maxey Flats radioactive waste disposal site exists as dissolved species, primarily complexes of the tetravalent ion with strong organic ligands such as ethylenediaminetetraacetic acid. The complexes are not sorbed well by sediment and are only partly precipitated by ferric hydroxide. These results indicate the importance of isolating radioactive waste from organic matter.
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